1. Field of the Invention:
This invention relates to a method of and apparatus for controlling an automatic transmission for an automobile by the use of a microcomputer or the like.
2. Description of the Prior Art:
When an automobile travels at a high altitude, the output of the engine is reduced as the atmospheric pressure is low. In a manual transmission, the driver carries out a down shifting operation from a high speed stage to a low speed stage in response to the reduction of the output of the engine to thereby compensate for the output reduction of the engine. However, in prior automatic transmissions, since the speed change pattern is predetermined by the vehicle speed and the opening of a throttle valve in an intake system irrespective of the reduction of the atmospheric pressure, the vehicle speed is changed according to the same speed change pattern as that at lower altitudes to degrade maneuverability even when the automobile moves from lower to higher altitudes.
Also, in an electronic fuel injection control engine which computes a fuel injection amount on the basis of intake air flow and operates a fuel injection valve to supply fuel on the basis of the computational result, air density varies with the altitude at which the automobile travels and the output characteristics of an air flow meter detecting intake air flow varies with the accumulation of stain on an intake wall of the air flow meter. Hence, it is necessary for maintaining the air fuel ratio of the mixture in a combustion chamber at a predetermined value to calculate a compensation value for the change in the altitude or output characteristics of the air flow meter and to compensate for the fuel supply on the basis of this compensation value. Further, to prevent evaporated fuel in a fuel tank from being purged to the atmosphere in an engine wherein the evaporated fuel is adsorbed by activated charcoal adsorbent to be purged to the intake system during the running of the engine, the air fuel ratio the mixture, as indicated by the output of an air fuel ratio sensor, varies also with an amount of purged evaporated fuel in addition to the fuel supply from the fuel injection valve. In the ordinary computational method of said compensation value, the feedback air fuel ratio (representing the actual air fuel ratio in the combustion chamber of the engine) is calculated on the basis of a feedback signal from the air fuel ratio sensor to adjust said compensation value on the basis of the deviation of the feedback air fuel ratio from the base air fuel ratio. However, when the running of the engine is once stopped and then resumed, the air fuel ratio sensor takes a predetermined time to be properly heated and produce the effective output. Thus, in this predetermined time and low temperature of the engine, the feedback signal from the air fuel ratio sensor is cut off and the fuel injection amount is calculated by open loop control. Also, in this predetermined time and the low temperature of the engine the purging of evaporated fuel to the intake system is stopped and the fuel injection amount, ignition timing, etc. are compensated on the basis of the final compensation value in the previous running of the engine. Accordingly, the adjustment of the compensation value caused by the purging of the evaporated fuel to the intake system must be avoided.
In the prior computational method of such a compensation value, the running condition of the engine is divided into a plurality of regions for example, according to the intake air flow. A complementary RAM (Random Access Memory) provided for each region stores a compensation value figured out in each region on the basis of the deviation of the feedback air fuel ratio from the base air fuel ratio, so that the number of the individual complementary RAMs is increased. Also, in the prior computational method, the deviation of the feedback air fuel ratio from the base air fuel ratio has a limit in each region, and when the deviation exceeds this limit the output of the air fuel ratio sensor is judged to be affected by the purging of the evaporated fuel to the intake system for adjusting the compensation value with the limit value. When the automobile moves from a low to a high altitude with high temperature and speed (condition under which a great amount of evaporated fuel is produced from the fuel tank), the deviation always exceeds the limit so that the change in the feedback air fuel ratio caused by the change in the atmospheric pressure is neglected and no altimetric compensation is accomplished by the prior method. In another prior method by which the altimetric compensation is performed, a path through which the evaporated fuel is purged to the intake system is closed by an electromagnetic valve to provide a period in which the purging of the evaporated fuel is stopped and the compensation value is degraded since the frequency of adjustment of the compensation value is reduced.